Compact fiber and diode sources of ultrashort optical pulses have not been able to produce significant pulse energies compared with their large-frame solid-state counterparts. The pulse energies from diode and fiber lasers vary typically between several picojoules and the nanojoule level, at best. These limited energies are not sufficient for a broad range of practical applications in which the use of compact ultrashort-pulse sources is highly desirable.
Potentially, much higher pulse energies can be extracted from rare-earth-doped fiber amplifiers. For example, the saturation energies of single-mode erbium-doped fiber amplifiers (EDFA's) are approximately 1 .mu.J. However, at such energies the peak power of the amplified ultrashort pulses would become unacceptably high for a single-mode fiber (approximately 1 MW for a 1-ps pulse). If light of such high power were confined in the small core of a fiber, high peak intensities would result, which would inevitably lead to strong nonlinear effects and pulse breakup. The only way to avoid this problem is to maintain sufficiently low peak powers in the amplifier, i.e., to amplify stretched, relatively long pulses. Provided that the initial pulses have a broad bandwidth and are suitably chirped, a short pulse duration can be attained by linear compression of the amplified pulses (e.g., by use of diffraction gratings).
Chirped pulse amplification methods are used in most of the laser systems producing high energy ultrashort pulses. For the last decade, chirped pulse amplification systems have relied on bulk diffraction-grating stretchers and compressors. The idea of chirped pulse amplification is that an ultrashort optical pulse is stretched prior to amplification and then recompressed back to its original width after the amplification is completed. This processing allows for a reduction in the distortion of the ultrashort pulses in the amplifier, and high pulse energies while maintaining short pulse durations. Recently, chirped long pulses were generated directly from a tunable laser source prior to amplification, and ultrashort high energy pulses were obtained by compressing the amplified pulses. A detailed discussion of this technique can be found in Galvanauskas et al., "Hybrid diode-laser fiber-amplifier source of high-energy ultrashort pulses," 19 Optics Letters 1043 (1994), which is hereby incorporated by reference. Although this method eliminates the need for the grating stretcher, it still requires a diffraction grating compressor.
In conventional systems, diffraction grating stretchers and compressors were the only type of dispersive delay line suitable for practical chirped pulse amplification systems. By using different configurations, both negative and positive dispersions can be attained. The magnitude of the dispersion is sufficient to stretch/recompress optical pulses by tens and hundreds of times, i.e., from femtoseconds to tens and hundreds of picoseconds. Such diffraction grating arrangements can handle pulses with very high energies without any pulse distortion due to nonlinear optical effects.
However, such dispersive delay lines have several major drawbacks: diffraction grating arrangements are polarization sensitive and typically large (up to a few meters long), have limited robustness and energy throughput (due to the diffraction losses), and distort the profile of the output beam. These features are particularly undesirable in compact fiber and laser-diode based chirped pulse amplification systems.
Bragg gratings have been used for various other purposes in optical signal processing, such as acousto-optical filters, as discussed in Tamir, Integrated Optics (Springer-Verlay New York, 1979) (hereby incorporated by reference) and as dispersive elements. Bragg gratings can be fabricated in optical fibers, in integrated optical waveguide structures, and in bulk materials. A Bragg grating in a germanosilicate fiber can be formed in the core via a light-induced periodic refractive-index change. The grating can be directly patterned from the side of a fiber using ultraviolet light, as discussed in Meltz, "Formation of Bragg gratings in optical fibers by a transverse holographic method," Optics Letters, Vol. 14, No. 15, Aug. 1, 1989, p. 823, the disclosure of which is hereby incorporated by reference. Using photosensitivity enhancing techniques, gratings can be written in any germanosilicate fiber, including standard telecommunications fibers. Optical waveguide gratings can be directly grown using a semiconductor material, and are most often used as integral parts of semiconductor laser structures (e.g., distributed Bragg reflector DBR or distributed feedback DFB laser diodes). However, other materials (e.g., LiTaO.sub.3 or LiNbO.sub.3) are also used for integrated waveguide structures including various grating structures. An example of a bulk Bragg grating is an acoustical optical filter. By chirping the FR modulating electric waveform, a chirped Bragg grating can be obtained.
Recently chirped Bragg gratings have also been used to compensate for dispersion in optical waveguides, as illustrated in FIG. 1 and described in detail in Ouellette, F. "Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides." Optics Letters Vol. 12, No. 10, October 1987, p. 847, hereby incorporated by reference. Since optical waves travel at rates which depend on their frequencies, different frequency waves reach a given destination at different times, thus creating a dispersion problem. By installing Bragg grating filters in the waveguide, this dispersion problem can be greatly reduced.
As discussed in Ouellette, the wavelength .lambda..sub.B of an optical wave reflected from a periodic Bragg structure is .lambda..sub.B =2n.LAMBDA., where .LAMBDA. is the period and n is the refractive index of the structure. If the period of such a structure varies along the grating, optical waves with different wavelengths are reflected at different positions. This gives a wavelength dependent delay .tau..sub..lambda.: EQU .tau..sub..lambda. =2L/.nu..sub.g
Here V.sub.g is the group velocity of light in the structure and L is the distance an optical wave of wavelength .lambda. penetrates into the Bragg grating. Therefore, the length of the Bragg grating determines the maximum delay difference between two different wavelengths and the magnitude of the grating period variation (grating chirp) determines the reflection bandwidth of the structure. The maximum .tau..sub..lambda. for a few-centimeter-long structure can be hundreds of picoseconds and the bandwidth can reach a few tens of nanometers.
While fiber Bragg gratings have been used as optical filters and chirped fiber Bragg gratings have also been used dispersion compensating components dispersive elements, the amplification of ultrashort optical pulses has been limited to using large and inefficient diffraction grating stretchers and compressors.